1. Field of the Invention
This invention relates to a charge pump circuit, specifically to a charge pump circuit with large output current capacity used for a power supply circuit and the like.
2. Description of the Related Art
Video equipment in recent years such as a camcorder, a digital still camera (DSC) and a mobile phone with DSC use CCDs (charge-coupled devices) to capture an image. A CCD drive circuit for driving the CCDs requires a power supply circuit that provides both positive and negative high voltages (over 10 volts) and a large current (several milliamperes). A switching regulator has been used for that purpose.
The switching regulator can generate a high voltage with high performance, i.e. with high power efficiency (output power/input power). However, it has a drawback to generate a harmonic noise when switching a current. Therefore, the power supply has to be used with a noise shield. In addition to that, it requires a coil as an external part.
Against this backdrop, attention is being given to a Dickson charge pump circuit as a power supply circuit for portable equipment in recent years. The Dickson charge pump device is described in detail in a technical journal “John F. Dickson ‘On-chip High-Voltage Generation in MNOS Integrated Circuits Using an Improved Voltage Multiplier Technique’, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. SC-11, NO. 3, pp. 374–378, JUNE 1976”, for example.
FIG. 11 shows a circuit diagram of a four-stage Dickson charge pump device. Diodes D1–D5 are connected in series. Each of coupling capacitors C1–C4 is connected to each of connecting nodes of the diodes D1–D5. CL refers to an output capacitor. CLK and CLKB are input clock pulses having opposite phase to each other. The CLK and CLKB are inputted to a clock driver 51. A numeral 52 refers to a current load. The clock driver 51 is provided with a power supply voltage Vdd. Herewith, an amplitude of the clock pulses Φ1 and Φ2 outputted from the clock driver 51 becomes Vdd approximately. The clock pulse Φ1 is fed to the capacitors C2 and C4, while the clock pulse Φ2 is fed to the capacitors C1 and C3.
In a stable state, in which a constant current Iout flows out, an input current to the charge pump circuit is a sum of a current from an input voltage Vin and a current provided from the clock driver. These currents are as described below, disregarding charging and discharging currents to and from stray capacitances. During a period of Φ1=High and Φ2=Low, an average current of 2 Iout flows through each of paths in directions depicted in the figure as solid line arrows.
During a period of Φ1=Low and Φ2=High, an average current of 2 Iout flows through each of paths in directions depicted in the figure as dashed line arrows. An average current of each of these currents over a clock cycle is Iout. A boosted voltage Vout from the charge pump device in the stable state is expressed by a following equation (1),Vout=Vin−Vd+n(Vφ′−V1·Vd)  (1)
where Vφ′ refers to an amplitude of a voltage at each of the connecting nodes induced through the coupling capacitor by a change in the clock pulse. V1 denotes a voltage drop due to the output current Iout and Vin denotes the input voltage which is usually set at Vdd in positive voltage boosting and at 0V in negative voltage boosting. Vd refers to a forward bias diode voltage, and n denotes a number of stages of pumping. Furthermore, V1 and Vφ′ are expressed by following equations,V1=Iout/(f(C+Cs))=(2 Iout T/2)/(C+Cs)Vφ′=V(C/(C+Cs)
where C denotes capacitance of each of the coupling capacitances C1–C4, Cs denotes a stray capacitance at each of the connecting nodes, Vφ denotes the amplitude of the clock pulses, f denotes a frequency of the clock pulses and T denotes a clock period of the clock pulses. Power efficiency η of the charge pump device is expressed by a following equation, disregarding charging and discharging currents from/to the clock driver to/from the stray capacitors and assuming Vin=Vdd.η=Vout Iout/((n+1) Vdd Iout)=Vout/((n+1)Vdd)
In this way, the Dickson charge pump circuit boosts the voltage by successively transferring electric charge to a next stage using the diodes as charge transfer devices. Although the Dickson charge pump circuit has advantages of no need for the coil and low noise, it also has disadvantage of incapability to provide large output current because of its low efficiency.
With this being the situation, the inventors have improved the Dickson charge pump circuit and have developed a charge pump circuit with high efficiency capable of providing large output current (several milliamperes). The improved charge pump circuit adopts MOS transistors for charge transfer instead of the diodes and has a level shift circuit to provide gates of the MOS transistors for charge transfer with level-shifted high voltage clocks to reduce ON resistance of the MOS transistors for charge transfer.
The improved charge pump circuit is described in a Japanese patent document Kokai (unexamined patent publication) No. 2001-286125.
However, an inrush current has presented a problem in putting the improved charge pump circuit into practical use. The coupling capacitors are not provided with sufficient amount of charge at the beginning of operation of the charge pump circuit. Each of the coupling capacitors is charged with enough amount of charge only after predetermined length of time after an input power supply is applied to the charge transfer devices of the charge pump circuit and the clock driver is put into operation. Thus a large inrush current ranging from 100 mA to 1 A flows from the input power supply and a power supply of the clock driver for duration from start of operation of the charge pump circuit until the charge pump circuit reaches a steady state of operation. A stabilized power supply is generally used as a power supply of the charge pump circuit. i.e. the input power supply and the power supply of the clock driver, while the stabilized power supply provides other circuits in the system with power supply.
Therefore when too large inrush current flows through the charge pump circuit, the stabilized power supply is made unstable, the other circuits malfunction, or a protection circuit of the stabilized power supply is activated, resulting in stopping operation of the other circuits.